Integrated assessment of mercury-tolerant plant-associated bacteria from a chlor-alkali site identifies candidates for bioaugmentation

Mercury (Hg)-contaminated environments harbour microorganisms with traits that may be useful for plant-assisted remediation, but high Hg tolerance alone may not identify the most suitable strains for inoculation. In this study, plant-associated bacteria isolated from a chlor-alkali site were evaluated through phenotypic screening, whole-genome analysis, genome-scale metabolic modelling, and willow inoculation assays. Fifty-one Hg-tolerant isolates were recovered, of which 16 were selected for detailed characterization. All selected isolates displayed at least one plant growth-promoting trait, including indole-3-acetic acid production, siderophore production, or phosphate solubilization. Pseudomonas canadensis UFC_B020 and Pseudomonas umsongensis UFC_B021 grew at 200 µM Hg, the highest concentration tested. Genome-based analyses identified putative functions related to metal transport, membrane efflux, oxidative stress responses and plant-beneficial metabolism, while community-scale modelling predicted potential metabolite exchanges within three selected bacterial consortia. In willow inoculation assays, consortium 1 increased root projected area by 5.7% relative to the uninoculated control. The strongest root response was therefore associated with a different consortium from that containing the two isolates with the highest Hg tolerance, highlighting the value of considering both Hg tolerance and plant responses in candidate selection. Together, these results support an integrated strategy combining Hg tolerance, plant growth-promoting traits, genome-based functional information and consortium performance to prioritize site-derived bacteria for subsequent bioaugmentation studies.